This invention relates to a device for stirring liquid in a storage tank while moving around in the tank, thus redissolving sludge which has separated out from the liquid, and preventing the accumulation of sludge on the bottom of the tank. It also relates to a method for determining the position of the stirrer in the tank so that the motion of the stirrer may be controlled.
In large tanks used for storage of liquid, (for instance, crude oil, residual oil, etc.) sludge may gradually separate out from the liquid over long periods of time. If this sludge is allowed to build up on the bottom of the tank, the quantity of liquid which can be stored in the tank is effectively reduced, and the tank must therefore be cleaned regularly to remove accumulated sludge. In order to clean the tank, it is necessary to drain off all the liquid. A device was therefore developed to obviate this necessity, consisting of a stirrer rotor inside the tank rotated by a motor outside the tank, which stirred the liquid and prevented sludge from separating out. The position of these stirrer rotors was however fixed, which meant that similar device had to be installed at a large number of points around the circumference of the tank. This solution is not practical and does not permit efficient stirring in the center area of the tank.
In order to solve this problem, the Applicant, in Japanese Patent Publication No. 59-46658 and Japanese Patent Provisional Publication No. 58-214383, proposed a device which stirred the liquid while moving around in the tank. This stirrer has rotors which permit it to move up and down, and rotors which permit it to move in a horizontal direction. By controlling these rotors from outside, the stirrer can be made to move around freely inside the tank. The liquid in the tank can therefore be stirred uniformly, sludge can be redissolved, and sludge can be prevented from accumulating anywhere on the bottom of the tank.
In order to lighten the weight of the stirrer and enable it to move smoothly through the liquid, it was suspended by a rope from a float to give it buoyancy. However, as the stirrer was suspended from the float, the stirrer sometimes lost its balance when it was moved horizontally or rotated, and sometimes collided with the tank inner wall.
Further, in order to move the stirrer to a desired location somewhere in a large tank by means of an external operation, it is necessary to determine the actual position of the stirrer at any time. As liquid storage tanks are usually completely enclosed, however, the position of the stirrer cannot be confirmed from outside.
The Applicant, in Japanese Patent Provisional Publication No. 58-213210, proposed a device for determining the position of the stirrer wherein a wire wound around a drum was suspended from the roof of the storage tank and connected to the upper part of the stirrer, the direction and length of wire released, as measured by a potentiometer or other means, being used to determine the position of the stirrer.
The structure of this device is however complex, and if the direction and length of wire released are not measured with a high degree of precision, it is difficult to determine the position of the stirrer with accuracy. Further, as the wire is paid out over a long distance, it sometimes goes slack or becomes entangled with the electric cable of the stirrer.